\Bibitem{Geg39} \by I.~G\'egalkine \paper Sur l'Entscheidungsproblem \jour Sb. Math. \yr 1939 \vol 48 \issue 2 \mathnet{http://mi.mathnet.ru/eng/sm5821} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=1932} \zmath{https://zbmath.org/?q=an:0022.19305|65.0027.02}